"The group of philosophical ideas that concerns us has been called essentialism by Popper, who has traced the impact of Plato's metaphysics on political thinking down to modern times. Even before Plato, Greek philosophy began to experience difficulties in dealing with change. If things grew, or passed away, they seem somehow unreal, suggesting that they belonged only to a world of appearances. Heraclitus, in adopting the notion that material things are illusory, maintained that all that really exists is "fire"—that is, process. ...To Plato, true reality exists in the essence, Idea, or eidos. ...In the hands of Aristotle, essentialist metaphysics became somewhat altered. ...[H]e held that [essences] did not exist apart from things. His works embraced the concepts of teleology, empiricism, and natural science... to understand a thing was to know its essence, or to define it. ...A true system of knowledge thus became essentially a classification scheme... Plato and Aristotle... both embraced the notion that ideas or classes are more than just abstractions—that is... both advocated forms of "realism." ...Aristotle ...advocated heirarchical classification... classes were differentiated... by properties held in common... An implication, of enormous historical importance, was that it became very difficult to classify things which change, or... grade into one another, or even to conceive or to discuss them. Indeed, the very attempt to reason in terms of essences almost forces one to ignore everything dynamic or transitory. One could hardly design a philosophy better suited to predispose one toward dogmatic reasoning and static concepts. The Darwinian revolution thus depended upon the collapse of the Western intellectual tradition."